The Friend murine erythroleukemia virus contains a replication defective leukemogenic viral component, the spleen focus-forming virus (SFFV), and a replicating helper virus component, the Friend murine lymphatic leukemia virus (F-MuLV). We have for the first time cloned SFFV free of F-MuLV by cell culture techniques, and have confirmed that it is replication defective and causes the rapid erythroleukemia induced by the Friend virus complex. By isolating SFFV free of helper virus in nonproducer cells, we have further been able to characterize the genome of SFFV by molecular hybridization; we have shown that it contains at least 2 discrete portions, one portion homologous to F-MuLV and another partially homologous to xenotropic virus. BIBLIOGRAPHIC REFERENCES: Troxler, D.H., Parks, W.P., Vass, W.C. and Scolnick, E.M.: Isolation of a fibroblast nonproducer cell line containing the Friend strain of the spleen focus forming virus. Virology 76: 602-615, 1977. Troxler, D.H., J.K. Boyars, W.P. Parks, and E.M. Scolnick: The Friend strain of spleen focus-forming virus: A recombinant between mouse type-C ecotropic viral sequences and sequences related to xenotropic virus. J. Virol. May, 1977 (in press).